Marco Rubio was favored by 15 percent of respondents in the latest poll, followed by Jeb Bush at 13 percent and Walker at 11 percent. No one else cracked double digits.

A month ago, Walker was at 18 percent, while Bush was next at 16 percent. Chris Christie and Mike Huckabee were tied for third at 8 percent each.

The latest poll was conducted after Hillary Clinton, Rubio, Ted Cruz and Rand Paul officially announced their candidates for the presidency.

Rubio fared best against Clinton in the latest survey with the former secretary of state at 45 percent to 43 percent for the Florida senator.

She led Walker 46-41.

Among national voters, Walker had a 24 percent favorable rating and 21 percent unfavorable rating; 54 percent didn't know enough about him to form an opinion.

The survey of 1,353 registered voters was conducted April 16-21 using live interviews over land lines and cell phones. The margin of error was plus or minus 2.7 percentage points. The survey included 567 Republicans, and that sample had a margin of error of plus or minus 4.1 percentage points.